1. Field of the Invention
The present invention relates to a process for forming a corrosion resistant coating. More particularly, the present invention relates to a process for forming a corrosion resistant coating, which comprises applying a solvent-type coating composition onto a substrate and drying it to form a prime coat, and then finish-coating a solventless coating composition thereon.
2. Description of the Prior Art
For the protection of substrates such as tanks, bridges, steel-frame structures or pipes which need to be protected for a long period of time, various solvent-type and solventless coating compositions have been employed in various combinations for the primer coating and finish coating of such substrates.
For instance, taking into consideration the selectivity to the primer, etc., various combinations of coating compositions, such as oil type coating composition/phenol resin type coating composition/chlorinated rubber type coating composition, organic or inorganic zinc-rich paint/chlorinated rubber type coating composition, epoxy resin type coating composition/polyurethane type coating composition, and epoxy resin type coating composition/epoxy resin type coating composition, have been employed for the coatings.
However, even when a substrate is coated with such a coating system, it has been impossible to attain adequate protection of the substrate for an extended period of time by the coating if the substrate is subjected to a severe environment where water drops or ice always exists, as in the case of hydraulic pipes at a water-power plant. Namely, cool water is always circulated in the hydraulic pipes, and the outer surface thereof always have water drops thereby formed except for the winter time. Consequently, blisters are likely to form on the coating film in about 6 to 15 months after the application of the coating. On the other hand, in view of the rapid increase in the labour costs for the coating operation, it is desired to have a coating composition developed which has a long interval for recoating i.e. which has superior durability and is capable of protecting the substrate from corrosion over an extended period of time.
In general, corrosion of iron is known to occur when water and oxygen simultaneously exist on the surface of the iron substrate. Accordingly, it is considered possible to protect iron for a long duration with the coating film either by preventing one of the above components from penetrating through the coating film or by reducing the rate of the penetration.
However, none of the conventional coating systems provides a totally satisfactory combination of the finish coating composition with a primer coating composition in respect of the selectivity of the finish coating composition to the primer coating composition or in respect of the inter-layer adhesion.